Horizontally constructed continuous steam generator and method for the operation thereof

ABSTRACT

The invention relates to a continuous steam generator provided, in a duct for hot gas circulating in a substantially horizontal direction, with a continuous heating surface of an evaporator comprising a plurality of steam generator tubes which are mounted in parallel for circulating a fluid flow. The inventive device requires exceptionally low construction expenditures and ensures a high degree of safety and a high efficiency. For this purpose, the continuous heating surface of the evaporator is characterized in that it comprises a segment of the heating surface through which a moving fluid flows in an opposite direction with respect to the heated gas direction is positioned in such a way that a saturated steam temperature which is adjusted during operation at the exit from the continuous heating surface deviates from the predetermined maximum value of the temperature of the gas prevailing during operation at the outlet of the segment of the heating surface. In addition, one or several entry collectors are disposed at a close distance from the outlet of the heating surface on the gas side in such a way that the moving fluid has a flow speed in the downpipe which is higher than the minimum speed required for pulling nascent steam bubbles.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US National Stage of International ApplicationNo. PCT/EP2004/008644, filed Aug. 2, 2004 and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent application No. 03020022.4 EP filed Sep. 03, 2003, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a continuous steam generator wherein acontinuous evaporator heating surface comprising a plurality ofparallel-connected steam generator tubes providing a flow path for aflow medium is disposed in a hot gas duct through which hot gas can flowin an approximately horizontal direction.

BACKGROUND OF THE INVENTION

In a gas and steam turbine plant, the heat contained in the expandedworking medium or heating gas from the gas turbine is utilized for thegeneration of steam for the steam turbine. Heat transfer takes place ina waste-heat steam generator disposed downstream of the gas turbine andin which a number of heating surfaces for water preheating, steamgeneration and steam superheating are normally disposed. The heatingsurfaces are connected into the water/steam circuit of the steamturbine. The water/steam circuit normally contains several, e.g. three,pressure stages, in which case each pressure stage may have anevaporator heating surface.

For the steam generator mounted downstream of the gas turbine on theheating-gas side as a waste-heat steam generator, a number ofalternative design concepts are suitable, namely configuration as acontinuous steam generator or as a circulation steam generator. In thecase of a continuous steam generator, the heating of steam-generatortubes provided as evaporator tubes results in evaporation of the flowmedium in the steam generator tubes in a single pass. In contrast, inthe case of a natural or forced circulation steam generator, thecirculating water is only partly evaporated as it passes through theevaporator tubes, the water that is not evaporated being is re-fed tothe same evaporator tubes for further evaporation after separation ofthe generated steam.

A continuous steam generator, in contrast to a natural or forcedcirculation steam generator, is not subject to any pressure limitation,which means that live-steam pressures well above the critical pressureof water (P_(cri)221 bar)—where there is only a slight difference indensity between a fluid-like medium and a steam-like medium—arepossible. A high live steam pressure promotes high thermal efficiencyand therefore low CO₂ emissions from a fossil-fired power plant. Inaddition, a continuous steam generator has a simple type of constructioncompared with a circulation steam generator and can therefore bemanufactured particularly inexpensively. The use of a steam generatordesigned according to the continuous principle as the waste-heat steamgenerator of a gas and steam turbine plant is therefore particularlyadvantageous for achieving a high overall efficiency of the gas andsteam turbine plant using a simple type of construction.

Particular advantages in terms of manufacturing costs, but also ofmaintenance required, are provided by a horizontally constructedwaste-heat steam generator in which the heating medium or heating gas,i.e. the exhaust gas from the gas turbine, is passed through the steamgenerator in an approximately horizontal flow direction. However, with ahorizontally constructed continuous steam generator the steam generatortubes of a heating surface may be subjected to markedly differentialheating depending on their positioning. Particularly in the case ofsteam generator tubes connected to a common header on the output side,differential heating of individual steam generator tubes may result in acombining of steam flows with greatly differing steam parameters andtherefore undesirable efficiency losses, in particular comparativelydiminished effectiveness of the heating surfaces affected andconsequently reduced steam generation. Differential heating of adjacentsteam generator tubes may also result in damage to the steam generatortubes or the header, particularly in the region where they dischargeinto headers. The per se desirable use of a horizontally constructedcontinuous steam generator as a waste-heat steam generator for a gasturbine can therefore entail considerable problems in terms ofsufficiently stabilized flow control.

EP 0 944 801 B1 discloses a steam generator suitable for a horizontaltype of construction and additionally having the abovementionedadvantages of a continuous steam generator. To this end, the disclosedsteam generator is designed in respect of its continuous evaporatorheating surface in such a way that a steam generator tube heated morethan another steam generator tube of the same continuous evaporatorheating surface has a higher throughput of flow medium than the othersteam generator tube. If differential heating of individual steamgenerator tubes occurs, the continuous evaporator heating surface of thesteam generator disclosed therefore exhibits, in its flow characteristictypical of a natural circulation evaporator heating surface (naturalcirculation characteristic), a self-stabilizing behavior resulting in amatching of the outlet-side temperatures even to differentially heatedsteam generator tubes connected in parallel on the flow medium sidewithout the need for external intervention. However, this conceptrequires that the disclosed steam generator be designed for feeding withflow medium having comparatively low mass flow density.

SUMMARY OF THE INVENTION

The object of the invention is therefore to specify a continuous steamgenerator of the abovementioned type which ensures particularly highoperational reliability even when fed with flow medium withcomparatively high mass flow densities. In addition, a particularlysuitable method for operating the steam generator of the abovementionedtype shall be set forth.

To achieve this object with respect to the continuous steam generator,the continuous evaporator heating surface comprises a first heatingsurface segment through which the flow medium can flow countercurrentlyto the heating gas duct and another heating surface segment connectedupstream of said heating surface segment on the flow medium and heatinggas side, the flow-medium-side outlet of the first heating gas segmentviewed in the heating gas direction being positioned such that thepressure-dependent saturated steam temperature arising at the outlet ofthe continuous evaporator heating surface during operation deviates byless than a predefined maximum deviation of no more than 70° C. from theheating gas temperature obtaining at the position of the outlet of theheating surface segment during operation.

The invention is based on the consideration that, if the continuousevaporator heating-surface is fed with comparatively high mass flowdensities, locally differential heating of individual tubes could affectthe flow conditions in such a way that less flow medium flows throughmore strongly heated tubes and more flow medium flows through lessstrongly heated tubes. More strongly heated tubes would in this case becooled worse than less strongly heated tubes, with the result that thetemperature differences occurring would be automatically amplified. Inorder to be able to effectively meet this eventuality even withoutactively influencing the flow conditions, the system must be suitablydesigned for fundamental and total limiting of possible temperaturedifferences. To this end, the knowledge can be used that, at the outletfrom the continuous evaporator heating surface, the flow medium musthave at least the saturated steam temperature essentially due to thepressure in the steam generator tube. On the other hand, however, theflow medium can have a temperature no higher than that of the heatinggas at the point of outlet of the flow medium from the continuousevaporator heating surface. By suitably matching these two temperaturelimits generally defining the possible temperature interval, the maximumpossible temperature imbalances can therefore also be suitably limited.By subdividing the continuous evaporator heating surface into anoutlet-side counterflow segment and another segment upstream of it onthe heating gas and media side, the outlet is freely positionable in theheating gas direction, with the result that an additional designparameter is available, a particularly suitable means of matching thetwo temperature limits being the selective positioning of the outlet ofthe continuous evaporator heating surface in the flow direction of theheating gas.

The positioning of the outlet of the continuous evaporator heatingsurface in relation to the temperature profile of the heating gas in thegas flue is advantageously selected such that a maximum deviation ofapproximately 50° C. is maintained so as to ensure particularly highoperating reliability in respect of available materials and furtherdesign parameters.

A particularly simple and therefore also robust type of construction canbe achieved by making the heating surface particularly simple in respectof collecting and distributing the flow medium. To this end, the heatingsurface is suitably implemented for performing all the process steps ofcomplete evaporation, i.e. pre-heating, evaporation and at least partialsuperheating, in a single stage, i.e. without interposed components forcollecting and/or distributing the flow medium. A number of steamgenerator tubes therefore advantageously comprise a plurality of riserand downcomer tube sections connected in series in an alternating manneron the flow medium side.

In this arrangement heating takes place both in the riser and downcomertube sections. However, such a connection of steam generator tubes inwhich heating of downflow tube sections also takes place generallyinvolves the risk of flow instabilities occurring. It has been foundthat the occurrence of steam bubbles in downflow steam generator tubesmay be regarded as one of the possible causes of this. If steam bubbleswere to form in a downflow steam generator tube, they could rise in thewater column present in the steam generator tube, thereby performing amovement counter to the flow direction of the flow medium. In order toconsistently prevent any such movement of steam bubbles possibly presentagainst the flow direction of the flow medium, forced entrainment of thesteam bubbles in the actual flow direction of the flow medium must beensured by suitable specification of operating parameters. This can beachieved by arranging that the continuous evaporator heating surface isfed in such a way that the flow rate of the flow medium in the steamgenerator tubes has the desired entrainment effect on any steam bubblespresent. A comparatively high flow rate even in the first downflow steamgenerator tube can be achieved in a very simple manner by means ofcomparatively strong heating of the steam generator tubes at theflow-medium-side inlet and the resultant rapid increase in the steamcontent of the flow medium. For this purpose the flow-medium-side inletof the continuous evaporator heating surface is advantageouslyimplemented as a riser tube section and disposed close to theheating-gas-side inlet of the continuous evaporator heating surface insuch a way that, during operation, the flow medium flowing through thesteam generator tubes has a flow rate higher than a predefined minimumrate at the inlet of the first downcomer tube section.

The first riser and downcomer tube sections preferably constitute anadditional heating surface segment disposed in a cocurrent flowconfiguration, hereinafter also referred to as a cocurrent segment,which advantageously precedes, on the flow medium side, the heatingsurface segment advantageously disposed in a countercurrent flowconfiguration, hereinafter also referred to as a countercurrent segment.By means of such an arrangement of the segments in the heating gas duct,the advantage of a pure countercurrent flow configuration, that ofeffectively transferring the heat of the exhaust gas to the flow medium,is largely retained while at the same time achieving a high inherentsafeguard against damaging temperature differences at theflow-medium-side outlet.

In an alternative advantageous embodiment, however, the additionalheating surface segment can also be connected countercurrently withrespect to the heating gas direction.

The steam generator is usefully employed as a waste-heat steam generatorof a gas and steam turbine system, the steam generator beingadvantageously connected downstream of a gas turbine on the heating gasside. In this arrangement, it is advisable for supplementary firing forincreasing the heating gas temperature to be disposed downstream of thegas turbine.

In respect of the method, the abovementioned object is achieved byeducting the flow medium from the continuous evaporator heating surfacein the heating gas direction at a position at which the heating gastemperature obtaining during operation deviates by less than apredefined maximum deviation of no more than 70° C. from the saturatedsteam temperature arising during operation as a result of the pressureloss in the continuous evaporator heating surface.

Upstream of its outlet from the continuous evaporator heating surface,the flow medium is advantageously fed countercurrently to the heatinggas, a maximum deviation of approximately 50° C. being specified in anadditional or alternative advantageous embodiment.

In order to consistently prevent any flow instabilities from occurring,the flow medium is advantageously exposed to strong heating at orimmediately after the inlet to the continuous evaporator heating surfacein such a way that it exhibits a flow rate of more than a specifiedminimum rate in a first riser tube section of the relevant steamgenerator tube.

Advantageously the flow rate required for the entrainment of steambubbles produced in the relevant first downcomer tube section ispredefined. The continuous evaporator heating surface is therefore fedin such a way that, even in the first downflow steam generator tube, thecomparatively high flow rate has the desired entrainment effect on anysteam bubbles present, thereby reliably preventing flow instabilitiescaused by any movement of rising steam bubbles against the flowdirection of the flow medium.

The advantages achieved with the invention are in particular that, bymeans of the now provided positioning of the flow-medium-side outlet ofthe continuous evaporator heating surface, adapted to the temperatureprofile of the heating gas in the gas flue, the overall achievabletemperature interval between saturated steam temperature of the flowmedium and heating gas temperature at the point of outlet duringevaporation of the flow medium is comparatively tightly limited, so thatonly small outlet-side temperature differences are possible irrespectiveof the flow conditions, thereby ensuring adequate matching of thetemperatures of the flow medium in every operating state. However, it isalso ensured, moreover, that the possible outlet temperatures arelimited in absolute terms, so that they remain reliably within thepermissible temperature limits predefined by the material properties.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the invention will now be explained ingreater detail with reference to the accompanying drawing. Said FIG is asimplified view in longitudinal section of a horizontally constructedcontinuous steam generator.

DETAILED DESCRIPTION OF THE INVENTION

The continuous steam generator 1 according to the FIG is connecteddownstream of a gas turbine (not shown) on the exhaust gas side in themanner of a waste-heat steam generator. The continuous steam generator 1has a surrounding wall 2 which forms a heating gas duct 6 for theexhaust gas from the gas turbine, heating gas flowing through said duct6 in an approximately horizontal direction x indicated by the arrows 4.In the heating gas duct 6 there are disposed a number of heatingsurfaces designed according to the continuous principle, also termedcontinuous evaporator heating surface 8. Although only one continuousevaporator heating surface 8 is shown in the example depicted in theFIG, a larger number of continuous evaporator heating surfaces can alsobe provided.

The evaporator system formed by the continuous evaporator heatingsurface 8 can be impinged by flow medium W which evaporates in a singlepass through the continuous evaporator heating surface 8 and, on leavingthe continuous evaporator heating surface 8, is educted as steam D andgenerally fed to superheater heating surfaces for further superheating.The evaporator system formed by the continuous evaporator heatingsurface 8 is connected into the water-steam circuit of a steam turbine(not shown in greater detail). In addition to the evaporator system, thewater-steam circuit of the steam turbine contains a number of otherheating surfaces not shown in greater detail in the FIG. The heatingsurfaces can be e.g. superheaters, medium pressure evaporators,low-pressure evaporators and/or economizers.

The continuous evaporator heating surface 8 of the continuous steamgenerator 1 according to the FIG comprises, in the manner of a tubebundle, a plurality of parallel-connected steam generator tubes 12providing a flow path for the flow medium W. A plurality of steamgenerator tubes 12 is disposed side by side viewed in the heating gasdirection x, only one of the thus disposed steam generator tubes 12being visible. On the flow medium side, the steam generator tubes 12thus disposed side by side are preceded upstream of their inlet 13 tothe heating gas duct 6 by a common inlet header 14 and followeddownstream of their outlet 16 from the heating gas duct 6 by a commonoutlet header 18. The steam generator tubes 12 comprise a plurality ofriser tube sections 20 through which flow medium W flows in the upwarddirection and downcomer tube sections 22 through which it flows in thedownward direction, these being interconnected by crossflow sections 24through which the flow medium W flows horizontally.

The continuous steam generator 1 is designed for particularly highoperating reliability and consistent suppression of significanttemperature differences (also termed temperature unbalance) at theoutlet 16 between adjacent steam generator tubes 12 even when the steamgenerator is fed with comparatively high mass flow densities. For thispurpose the continuous evaporator heating surface 8 comprises, in itsdownstream region viewed from the flow medium side, a heating surfacesegment 26 connected countercurrently to the heating gas direction x. Anumber of riser tube sections 20 and downcomer tube sections 22interconnected by crossflow sections 24 additionally form a furtherheating surface segment 28 connected cocurrently with the heating gasdirection x upstream of the heating surface element 26. Thisconfiguration means that the positioning of the outlet 16 is selectablein the heating gas direction x. This positioning can be selected for thecontinuous steam generator i in such a way that the pressure-dependentsaturated steam temperature of the flow medium W arising in thecontinuous evaporator heating surface 8 during operation deviates byless than a specified maximum deviation of approximately 50° C. from theheating gas temperature obtaining at the position or at the height ofthe outlet 16 of the heating surface segment 26 during operation. As thetemperature of the flow medium W at the outlet 16 must always be atleast equal to the saturated steam temperature, but on the other handmay be higher than the heating gas temperature obtaining at this point,the possible temperature differences between differentially heated tubescan be limited to the specified maximum deviation of approximately 50°C. without additional countermeasures.

The heating surface segment 28 disposed well upstream in the heating gasduct 6 in the heating gas direction x is therefore followed on theheating gas and flow medium side by the heating surface segment 26likewise formed from a number of riser tube sections 20 and downcomertube sections 22 interconnected by crossflow sections 24 and throughwhich the flow medium flows countercurrently to the heating gasdirection x.

An arrangement of tube sections though which flow medium flows in thedownward direction, like the downcomer tube sections 22 inside theheating gas duct 6, is basically only possible if the stability of theflow within the steam generator tubes 12 is ensured by suitablemeasures. Heating of tube sections through which flow medium flows inthe downward direction tends to result in the formation of steam bubblesin the flow medium W which, if they rise against the flow direction ofthe flow medium W because of their low specific gravity, may adverselyaffect flow stability and therefore the operational reliability of thecontinuous steam generator 1. On the other hand, a configuration of thesteam generator tubes 12 whereby only the tube sections through whichflow medium flows in the upward direction, i.e. the risers 20, areheated, involves high construction costs.

A particularly simple and therefore also robust type of construction ofthe continuous steam generator 1 can be achieved by making thecontinuous evaporator heating surface 8 particularly simple in respectof collecting and distributing the flow medium W and eliminatingadditional components such as collecting tubes. Instead of this, thesteam generator tubes 12 incorporate a plurality of alternating riser 20and downcomer tube sections 22 connected in series on the flow mediumside which are mounted inside the heating gas duct 6, i.e. subjected toheating by the heating gas.

The inlet 13 is disposed at the gas-side inlet of the continuousevaporator heating surface 8, i.e. in the heating gas duct 6 wellupstream in the heating gas direction x. By disposing the inlet 13 inthe area of the heating gas duct 6 in which the heating gas has thehighest temperature, very rapid heating and therefore evaporation of theflow medium W in the steam generator tubes 12 is achieved. As the flowrate of the water-steam mixture, the mass flow rate being equal, ishigher the greater the steam portion and therefore the specific volumeof the mixture, with this arrangement of the inlet header 14 the flowmedium W attains a high flow rate comparatively quickly.

This is particularly favorable in order to ensure the stability of theflow taking place in the steam generator tubes 12. An important factorseverely detrimental to flow stability is the occurrence of steambubbles in the steam generator tubes 12. Because of their low specificweight, gas bubbles forming may rise upward in the steam generator tubes12, thereby moving against the flow direction in the downcomer tubesection 22. As a movement of this kind would seriously impair flowstability, the rising of steam bubbles produced in the steam generatortubes 12 must be consistently prevented. An important criterion for flowstability is the flow rate of the flow medium W. If, in the first tubesection through which the flow medium flows in the downward direction,i.e. in the first downcomer 22, it already has a value at least as highas the rate required for entrainment of the steam bubbles, said bubbleswill be entrained with the flow and any rising movement contrary to theflow direction will be reliably eliminated. The positioning of the inlet13 at the heating-gas-side inlet and the resultant high flow rate of theflow medium W even in the first downcomer tube section 22 ensures thedesired entrainment effect on the steam bubbles forming, while at thesame time minimizing construction costs.

1-12. (canceled)
 13. A continuous steam generator, comprising: a heatinggas duct through which heating gas flows in an approximately horizontaldirection; a continuous evaporator heating surface disposed in theheating gas duct and comprising a number of parallel-connected steamgenerator tubes that provide a flow path for a flow medium; a heatingsurface segment incorporated with the continuous evaporator heatingsurface and through which the flow medium can flow countercurrently tothe heating gas duct; and an additional heating surface segment arrangedupstream of the heating surface segment on the flow medium and heatinggas side whose flow-medium-side outlet viewed in the heating gasdirection is positioned in such a way that the saturated steamtemperature arising at the outlet of the continuous evaporator heatingsurface during operation deviates by less than a predefined maximumdeviation of no more than 70° C. from the heating gas temperatureobtaining at the position of the outlet of the heating surface segmentduring operation.
 14. The continuous steam generator according to claim13, wherein a number of steam generator tubes incorporate a plurality ofalternating riser and downcomer tube sections connected in series.
 15. Acontinuous steam generator according to claim 13, wherein theflow-medium-side inlet of the continuous evaporator heating surface isdisposed close to the heating-gas-side inlet of the continuousevaporator heating surface in such a way that in operation the flowmedium passing through the steam generator tubes has a flow rate of morethan a predefined minimum rate.
 16. The continuous steam generatoraccording to claim 13, wherein the additional heating surface segment isconnected countercurrently to the heating gas direction.
 17. Thecontinuous steam generator according to claim 13, wherein the additionalheating surface segment is connected cocurrently with the heating gasdirection.
 18. The continuous steam generator according to claim 13,wherein a gas turbine is connected upstream on the heating gas side. 19.A method for operating a continuous steam generator, comprising:providing a heating gas duct through which heating gas flows in anapproximately horizontal direction and which has a continuous evaporatorheating surface comprising a number of parallel-connected steamgenerator tubes which provide a flow path for a flow medium; andeducting the flow medium from the continuous evaporator heating surfaceat a position, viewed in the heating gas direction, at which the heatinggas temperature obtaining during operation deviates by less than apredefined maximum deviation of no more than 70° C. from the saturatedsteam temperature arising at the outlet of the continuous evaporatorheating surface during operation.
 20. The method according to claim 19,wherein the flow medium upstream of its outlet from the continuousevaporator heating surface is supplied countercurrently to the heatinggas.
 21. The method according to claim 19, wherein, at or immediatelyafter the inlet to the steam generator tubes, the flow medium issubjected to strong heating such that it exhibits, in a first downcomertube section of the relevant steam generator tube, a flow rate of morethan a predefined minimum rate.
 22. The method according to claim 21,wherein the flow rate required for entraining steam bubbles produced inthe respective first downcomer tube section is predefined as the minimumrate.
 23. The method according to claim 19, wherein downstream of itsinlet to the continuous evaporator heating surface the flow medium issupplied countercurrently to the heating gas.
 24. The method accordingto claim 21, wherein, downstream of its inlet to the continuousevaporator heating surface the flow medium is fed cocurrently with theheating gas.